I am getting ready to D&T a 1903A3 rifle that someone has already started to sporterised. I have a one piece Leupold base marked 1903A3, and I am using a B-Square D&T jig.
My problem is this. I have been placing the base on the receiver to get an idea where the holes will be before I drill. What I am finding is that if I position the rear hole in the middle of the male dovetail on the rear receiver bridge, the front holes are too far back on the front receiver. It appears that the rear hole on the front ring, will be drilled into the upper locking lug.
If I move the base forward to get that rear screw moved forward out of the locking lug, the screw for the rear bridge moves off the dovetail. Plus the inlettng on the front of the base is tapered at the back end, so that it won't move too far forward.
Plus, the D&T jig has 2 locating pins(Mauser jig only uses one). With the rear pin in place, the jig cannot move far enough back to position the rear screw in the middle of the male dovetail as it should be. I can tap the rear pin up, so that I can move the jig to the rear, but that puts the front screw holes too far back.
I'm at a loss as to how to fix this.

I know everyone is scared to drill into a locking lug and should be, but sometimes and some receiver base combos require that the hole be over the locking lug and not the recess. This is common of 1917 enfield bases. Answer is they are meant to be a blind hole and never drilled into the lug itself. As long as you don't drill deep there is no issues that I can see placing a hole over (not into) the locking lug. Most of the time this bases were installed without removing the barrels and the gunsmith practiced the lost art of drilling and tapping a blind hole.
Don

I just measured the front screws on the base. It appears that I need to drill a hole that is about .165" deep. Right in the middle of the locking lug. I am not sure if I want to do that. I have a Weaver two piece base set on the way. I may go that route.

.165 is awful deep. 4 threads is more than enough support. 48 tpi so that's about .021 per thread x 6 threads (4 full 2 for bottoming tap) = .126
What is the diameter of the receiver ring and what is the diameter of the bolt at the lugs? Do the math, I doubt you will be near the where the lug bears. Even at .165 deep you won't be into the lug.
Don